The present invention relates to a method for drying a fiber web using a two wire and two row drying cylinder arrangement. The invention has particular use in a papermaking machine. The invention concerns supplying air flows at, through or near different parts of the loop path of the dryer wire in the path between adjacent drying cylinders in one row for effective web control. The invention concerns a method and an apparatus for the carrying out such a method related to the air flows.
The prior art includes U.S. Pat. No. 5,379,526 (FIG. 2) and Federal Republic of Germany 44 04 726 (FIG. 6).
Constructions in the above cited prior art have the following features in common:
1. Upon the web to be dried leaving a "first" drying cylinder, the web first travels a distance together with and is therefore supported by a first wire generally up to a wire guide roll that is off the row of the first drying cylinder wire and is toward the other row of drying cylinders. After leaving this guide roll, the web separates from the wire and travels unsupported to the following "second" drying cylinder which is part of the other row of cylinders. Thus, the "free web paths" or open draws, i.e., the web travel paths over which the web is not supported, are relatively short. This prevents the danger of fluttering of the web and tearing of the web. In regions where the paper web and/or the drying wire do not rest on a closed surface, for instance, a cylinder surface or a guide roll surface, they can dry more or less undisturbed.
2. The drying wire coming from the first drying cylinder in the first row wraps around the wire guide roll. There the web separates from the wire. Then the wire travels from the guide roll back to the next "third" drying cylinder in the first row of cylinders, thereby forming a loop of the wire or a pocket. This loop contains a so called air guide box.
3. A sealing element supported by the air guide box prevents a layer of boundary air from being drawn into the loop. That layer of boundary air is carried along by the wire traveling over the first cylinder.
4. A second region of the wire loop extends from the guide roll to the third drying cylinder. In the second region, the drying wire is not covered by the web. A second flow of air is required in the second region to ventilate the so called "pocket", i.e., the region between the drying wire and the web of paper that is spaced away from the wire and that is traveling onto the next, i.e., third cylinder. The second flow of air blows through the drying wire. This so called pocket ventilation is effected in FIG. 6 of Federal Republic of Germany '726 by means of a blower which is arranged in the "run-off gusset" between the wire guide roll and the departing wire.
In many cases, it has been found that the known method of producing a vacuum along the first part of the wire loop, between the first cylinder and the guide roll and/or the known method of ventilating the pocket are not sufficient.
The operating speeds of modern papermaking machines have greatly increased in recent years. Some are now operating at a speed of 1800 m/min. and more. This makes it necessary to increase the drying performance of each drying cylinder and thus to increase the amount of drying air necessary for removing the vapors that come off the web as it is dried. To do this, more air must be forced into the pocket through the uncovered drying wire. At the same time, despite the greater web travel speed, the web travel must be as stable and quiet as possible.